Nitrogen oxide (NOx) emissions contribute significantly to photochemical smog and also to acid rain. NOx includes both nitrogen oxide (NO) and nitrogen dioxide (NO.sub.2), both of which will be referred to as NOx. NOx is generated during the combustion of fossil fuels and a major generator of NOx is the diesel engine. Currently, new emissions standards for diesel engines are being proposed. For example, the European Euro-5 and the proposed US-2007 standards require a significant reduction in both NOx and particulate matter (PM) emissions.
In addition, diesel emissions have been classified as Toxic Air Contaminants (TACs) in the State of California. Under the Federal Clean Air Act, California must meet certain clear air requirements established by the Federal Government in order to qualify for federal highway funding. It is unlikely that those guidelines can be met without reducing emissions from mobile sources. Diesel mobile sources produce a disproportionate percentage of all emissions due to the inherent nature of the fuel and the engine.
In response, diesel engine manufacturers are developing systems to treat the exhaust stream of their diesel engines. Most of these solutions, however, make a clear trade off between emissions and fuel consumption. Some proposed systems are even associated with a distinct fuel penalty. Of course, fuel efficiency is extremely important, as the engine operator incurs an increased operational cost.
Another problem is that diesel engines typically last longer than other types of engines, and older engines produce more toxic emissions than newer engines.
Therefore, there exists a need for an emission control system that can reduce both NOx and PM emissions without incurring a fuel penalty, and that can be retrofitted to existing diesel engines.